Many processes have been described in which SO.sub.2 is absorbed in a buffered salt solution and the buffered salt solution is contacted with H.sub.2 S to form sulfur.
U.S. Pat. No. 2,729,543 discloses a process in which SO.sub.2 is absorbed in a solution of a salt capable of buffering in the range of pH 4-6 and the solution reacted with H.sub.2 S to form sulfur. A number of inorganic and organic salts are disclosed including alkali metal salts of citric acid. The patent discloses that the reaction rate between the solution and H.sub.2 S is pH and temperature sensitive and is at maximum at a temperature between about 40.degree. and 70.degree. C. The reaction rate is independent of the H.sub.2 S partial pressure but is dependent on the thiosulfate concentration. The SO.sub.2 loadings and sulfur formation rates disclosed are very low.
U.S. Pat. No. 3,833,508 discloses a method for selectively absorbing SO.sub.2 from a gas stream containing H.sub.2 S by contacting the gas stream with a solution of an alkali metal salt of certain organic acids such as citric acid at at a pH in the range of 4 to 6. The H.sub.2 S is not absorbed and does not react with the SO.sub.2 absorbed in the solution.
U.S. Pat. No. 3,911,093 discloses a regenerative process in which SO.sub.2 is absorbed in a buffered alkali metal orthophosphate solution. The solution containing the SO.sub.2 is reacted with H.sub.2 S to form sulfur and regenerate the absorbent solution which is recycled to absorb SO.sub.2. The process has been used to remove small amounts of SO.sub.2 from waste gases and to recover the sulfur values from "sour natural gas" which contains H.sub.2 S. The process requires large solution circulation rates since the orthophosphate solution has only a small capacity for absorbing SO.sub.2. In addition, the process has a difficulty in that sulfur continues to be formed after the solution has been removed from contact with the H.sub.2 S containing gas. An improved process is disclosed in U.S. Pat. No. 4,519,994 in which the amount of SO.sub.2 absorbed by the solution is increased by increasing the concentration of alkali metal phosphate in the solution. However, the amount of SO.sub.2 absorbed is still low. Other regenerative processes are disclosed in U.S. Pat. No. 3,983,225 and U.S. Pat. No. 4,442,083.
U.S. Pat. No. 2,031,802 and U.S. Pat. No. 4,366,134 disclose processes which use alkali metal phosphate and citrate salts to absorb SO.sub.2 from a gas stream followed by stripping the absorbed SO.sub.2 from the solution by heat or low pressure.
In the face of a need for removing SO.sub.2 from gas streams and recovering the sulfur values from sour gas, refinery streams, chemical process streams, coal gasification gas streams and coal liquefaction off gases, the buffered salt solution processes have not received wide commercial use despite their inherent advantage which include compact size, ease of control and low environmental impact. The lack of widespread commercial use of the processes is due to the large circulation liquid streams and the low sulfur formation rates which translate to high investment costs, and the difficulty with post-formation of sulfur which tends to form in the circulation solution after the solution has contacted H.sub.2 S and is being recirculated to the SO.sub.2 absorption apparatus. The post-formation of sulfur after contact with H.sub.2 S has been completed tends to clog conduits, heat exchangers and packing associated with the absorption of SO.sub.2.